Acoustical transducers convert electrical energy to acoustical energy, and vice-versa, and can be employed in a number of applications. In the detection of mobile vessels, for example, acoustic transducers are the primary component of sonar devices, and are generally referred to as projectors and receivers. Projectors convert electrical energy into mechanical vibrations that imparts sonic energy into the water. Receivers are used to intercept reflected sonic energy and convert the mechanical vibrations into electrical signals. Multiple projectors and receivers can be employed to form arrays for detecting underwater objects.
In a typical application, marine seismic vessels tow vibrators and discharge air guns, explosives and other acoustic projectors to generate seismic energy in marine geophysical testing. The seismic energy comprises a pressure pulse that travels through the water and underlying subsurface geologic structures. The energy is partially reflected from interfaces between the geologic structures and is detected with geophone or hydrophone sensors.
Conventional transducers, however, are associated with a number of unsolved problems. For instance, currently known transducer designs are generally not capable of producing large amounts of acoustical energy at low frequencies on the order of two kilocycles or less, and in particular, under 400 Hz. Similarly, there appears to be no transducer that operates with considerable efficiency so as to provide large power outputs over low frequency ranges.
Physical limitations on transducer design further complicate solving such deficiencies. For example, effective mechanical stress management is important for deep depth capability, as well as for the ability to produce high acoustic power levels.
Generally, the family of sonar projectors capable of generating low frequency operate in a wall flexure mode. These projectors include flextensionals, inverse flextensionals, bender discs, wall-driven ovals (also know as “WALDOs”), and slotted cylinder projectors. Slotted cylinders can typically operate at frequencies lower than the frequencies at which flextensionals and WALDOs can operate given a fixed wall thickness and effective diameter.
However, the achievable low frequency range of such slotted cylinders is still limited (nothing below 400 Hz), given the current need for low frequency transducers. Lower frequencies can be obtained by thinning the transducer wall thickness. On the other hand, as the wall thickness is decreased, mechanical stresses due to the wall flexure increase. For flextensionals and WALDOs to match the lower frequency capability of slotted cylinders, their walls would have to be thinned to a point where hydrostatic pressures would compromise their structural integrity.
What is needed, therefore, are robust, ultra-low frequency acoustic transducer designs.